Upgrade libgit2 to 1.5.0+

[craigds]

We've got an issue where a specific repo with a large packfile is unable to find specific objects - in this case the root tree of the head commit.

$ kart log
kart helper: unhandled exception
Traceback (most recent call last):
  File "kart/helper.py", line 236, in helper
  File "click/core.py", line 1130, in __call__
  File "click/core.py", line 1055, in main
  File "kart/cli_util.py", line 72, in invoke
  File "click/core.py", line 1657, in invoke
  File "click/core.py", line 1404, in invoke
  File "click/core.py", line 760, in invoke
  File "click/decorators.py", line 26, in new_func
  File "kart/log.py", line 259, in log
  File "kart/context.py", line 79, in get_repo
  File "kart/repo.py", line 460, in ensure_supported_version
  File "kart/repo.py", line 570, in table_dataset_version
  File "kart/tabular/version.py", line 64, in get_repo_wide_version
  File "kart/repo.py", line 668, in head_tree
KeyError: 'object not found - object is missing (ce5e0fef3cd848f93a03400221d2c11c7cb5c6a5)'

The object does exist in the repo - it can be read by git cat-file -p. fsck/gc don't seem to help.

Outside of kart, the object can be read by pygit2 1.11.1 (libgit2 1.5.0), but not by pygit2 1.9.1 (libgit 1.4.2).

I expect the upstream issue is libgit2/libgit2#6309 which was fixed in libgit2 1.5.0. The symptoms align (repo has >2GiB packfile and an MIDX)

Kart should upgrade to libgit2 1.5.0+ to fix the problem (and probably pygit2 too although I'm not sure if it matters)